1. Field of the Invention
This invention relates generally to mooring systems for offshore vessels and Floating Production Units (xe2x80x9cFPUsxe2x80x9d) such as Floating Storage and Offloading vessels (xe2x80x9cFSOsxe2x80x9d) and Floating Production Storage and Offloading vessels (xe2x80x9cFPSOsxe2x80x9d) and in particular to turret mooring arrangements, or systems, where a turret is rotatably supported on the vessel and where the turret is fixed to the sea bed by anchor legs so that the vessel can weathervane about the turret.
2. Description of the Prior Art
Turret mooring systems have been used for some time for FPUs and especially with FPSOs. FPSOs are production platforms typically constructed on tanker hulls. FPSOs are the most flexible of FPUs in terms of water depth and sea conditions due to their variation in moorings and ship shape configurations. FPSOs are spread moored (anchored directly to the sea floor), attached via an internal or external turret, which is moored to the sea floor or detachably secured to a separately floating buoy that is moored to the sea floor. FPSOs have excellent storage and topside facilities configurations due to their large size and ship shape. Further, many modem FPSOs are turret moored.
FPSOs compete with other kinds of floating production units such as semi-submersibles, spars, and tension leg platforms. Their competitiveness depends on their advantages and disadvantages.
As mentioned above, the present invention is directed to a turret mooring arrangement. Prior turret mooring arrangements are known in the art. For example, U.S. Pat. No. 3,335,690 to Busking shows a permanently anchored turret which is rotatably supported from a frame that extends from the bow of the vessel.
U.S. Pat. No. 4,650,431 to Kentosh illustrates a turret which can be disconnected from a frame secured from the bow of a vessel.
U.S. Pat. No. 4,490,121 to Coppens illustrates a disconnectable turret which is rotatably secured from the bow of the vessel.
U.S. Pat. No. 4,955,310 to Pollack illustrates a bearing system for mounting a turret on the outer beams of a vessel. The bearings shown in this patent allow the turret to pivot about upper and lower horizontal axes.
U.S. Pat. No. 5,515,804 to Pollack illustrates a bearing system for a turret with a generally rigid upper mount and including a resiliently deflectable support structure that includes a plurality of elastomeric shear pads.
U.S. Pat. No. 5,468,166 to Breivik et al. shows a disconnectable buoy which is receivable into a submerged receiving space of the vessel. The outer portion of the buoy is latched to the vessel, but has a central member of smaller diameter which is rotatably mounted in the outer member and has a through-going passage for hydrocarbon to be transported via the buoy. A flexible joint is provided at the top end of the central member. The flexible joint is secured to an inner part of a fluid swivel.
Identification of Objects at the Invention
A primary object of the present invention is to provide a less expensive turret mooring arrangement for a FPSO.
Another object of the present invention is to provide a turret mooring arrangement with a smaller turret than conventional FPSOs.
An additional object of the present invention is to provide a less expensive turret mooring arrangement for rotatably mounting a turret on a vessel under conditions of a vessel ovaling and moment loading on the upper axial/radial bearing.
The objects identified above along with other features and advantages of the invention are provided with a turret configuration for a low cost internal turret in which the turret includes an upper section, a lower section and a flex joint coupled therebetween. The turret mooring arrangement is rotatably supported on a vessel that floats at the surface of the sea and that can weathervane about the turret. The lower section of the turret is anchored by at least one mooring line which extends to the sea floor for anchoring the turret in a substantially geostationary position.
The upper section includes an axial/radial bearing assembly. This assembly permits the vessel to weathervane about the turret, yet resists other moment loadings caused by weather conditions, including sea conditions, causing the vessel to heave, pitch and yaw in the sea.
The flex joint is located just below the upper axial/radial bearing assembly. It is designed to minimize the effects of moment loading acting upon the upper axial/radial bearing assembly.
The lower section includes a lower radial bearing assembly comprising a lower radial bearing and lower bearing outer housing. The lower bearing outer housing is strategically shaped to correspond with the hull of the vessel or an outboard device attached to the vessel. Because the lower bearing outer housing is not permanently connected to the vessel or outboard device, no requirements exist to integrally fabricate the assembly to the vessel or outboard device. Instead, such an assembly can be fabricated separately, thereby reducing costs. Moreover, the shape of the lower radial bearing assembly housing can be more easily customized as desired or necessary to facilitate capturing of the turret mooring arrangement by the vessel. For example, the outer profile of the lower bearing outer housing may be polygonally shaped, frusto-conically shaped, or any other shape desired.